Durable mold of multilayer construction

ABSTRACT

A durable warp-free mold for casting molten metals comprising at least three different layers, each formed of a non-metallic refractory material having coefficients of thermal expansion decreasing from the external layer remote from the article to be molded to the internal layer closest to the article to be molded, said internal layer consisting essentially of silicon carbide, the intermediate layers consisting essentially of mixtures of silicon carbide and chamotte and said external layer consisting essentially of chamotte.

Waited States Patent Sakabe et a1.

1 1 Jan. 28, 1975 DURABLE MOLD OF MULTILAYER CONSTRUCTION Inventors: Koji Sakabe; Hideo lto; Nobuyuki Katura, all of Nishi, Japan Assignee: Sakabe Industry Co., Ltd.,

Aichi-ken, Japan Filed: Feb. 11, 1974 Appl. No.: 441,311

Related US. Application Data Continuation-impart of Ser. No. 206,053, Dec. 8, 1971, abandoned.

Foreign Application Priority Data BBQ-10,1970 BEL-13m:twat-m1451 09 2 164/41 Int. Cl. B221: 9/00 Field of Search 161/206, 162, 168, 166;

[ 56] References Cited UNITED STATES PATENTS 1,345,377 7/1920 Linbarger 161/166 2,943,008 6/1960 Saunders 161/162 Primary ExaminerGeorge F. Lesmes Assistant Examiner-S. S. Silverman [57] ABSTRACT 1 Claim, 2 Drawing Figures PATENTED JANZ 8 I975 FIG.

FlG.. 2

PATTERN DURABLE MOLD OF MULTILAYER CONSTRUCTION CROSSREFERENCE TO THE RELATED APPLICATION This is a continuation-in-part application of U.S. Ser. No. 206,053 filed on Dec. 8, 1971, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a durable mold for casting molten metals. Except for metal molds, the durable molds in use are, in most cases, moldings of highly heatresistant granular or powdery materials, that is, refractory molds.

Refractory molds are advantageous in that they resist pressures well, attain greater surface hardness than metal molds, and offer a free choice of the materials to form the molds. Thus, unless the component materials are wrongly chosen, molds totally inert to or unwettable with the metal being cast can be made. This will be all the more obvious to those skilled in the art when the use of a graphite mold for the casting of titanium alloy, a highly reactive metal, is taken for example.

However, the conventional durable molds of refractories which have hitherto been employed for the casting of steel, iron, and the like have a major disadvantage in common; they tend to suffer from warping or deformation due to expansion of the mold materials. Naturally the warping can seriously affect the dimensional accuracy of the molds and, in the case of split molds, make them eventually unusable due to leak-out. The distorsion with warpage is frequently presented as a problem in the fabrication of metal molds, too.

SUMMARY OF THE PRESENT INVENTION The present invention provides a mold so devised as to avoid warping, and more particularly what may be called a multilayer mold which consists ofa plurality of layers. For a better understanding of the present invention, an embodiment thereof consisting of three layers will be described below, though it should be clear that the invention is not limited thereto but is equally applicable to molds of more layers than three. The mold embodying the invention consists of an external layer, an intermediate layer, and an inner layer, formed of refractory materials with thermal expansion coefficients decreasing in the order mentioned. The warping which has inevitably occurred with molds of low-expansion refractories has for the first time been eliminated by the multilayer structure according to the present invention.

If it is now assumed that molten metal is poured into a conventional refractory mold, the inner surface of the mold in contact with the molten metal will be heated to a high temperature. As a result, the inner surface portion will expand sharply and cause ununiform distribution of heat throughout the mold, thus leading to warping, as will be clear to anyone skilled in the art. In accordance with the present invention, therefore, a mold is constructed of an inner layer which consists of a refractory material with a relatively low expansion coefficient, and the rest of layers having relatively high coefficients, the value of the external layer being the highest.

BRIEF DESCRIPTION OF THE DRAWINGS The method of fabricating the mold embodying the invention will now be described, by way of example,

with reference to the accompanying drawings in which: FIG. 1 is a cross-sectional view of a mold according to the invention as consisting of multiple layers, i.e., inner, intermediate, and external layers.

FIG. 2 is a schematic cross-sectional view ofa device for press forming the mold shown in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The external layer 1 need not be highly refractory but may be formed ofchamotte which is equivalent to grog, an ordinary refractory material. The intermediate layer 2 is formed of a mixture of silicon carbide and chamotte, and the inner layer 3 is formed of silicon carbide. As shown in FIG. 2, the mold was fast press formed, that is, placing a pattern with its patterned face upward, and then layers of powdery or granular materials for the inner, intermediate and external layers having compositions as shown in the following table 1 and having particle sizes as shown in table 2 were put on the pattern in that order and then a pressing force in a direction perpendicular to the layers was applied downwardly. The thicknesses of the layers were chosen so that after the press forming they were: 5 mm for the inner layer, 10 mm for the intermediate layer, and 30 mm for the external layer.

A friction press having a capacity of 150 kg/cm or more was used, and the pressing operation was made at normal temperature. The humidity of the powdary or granular materials was kept in the range of 3 to 7 percent. For the control of the expansion coefficients of the mold materials, suitable additive agent or agents for the increase or decrease of the coefficient may be added to the individual layers. In the example of the present invention fine powder of tridymite was used. It was added in amounts of 3 percent to the external layer, 0.7 to 0.8 percent to the intermediate layer and 0.5 percent to the inner layer, in order to control the expansion of the respective layers.

After press forming, the mold was taken out from the pattern and dried at a temperature of l20 C for 24 hours to remove the moisture therein. Then it was sintered at l,300C at the following temperatures for raising and keeping conditions followed by furnace cooling.

Normal temperature 4 400C 2 hours 400C 900C 3 do. 900C g 1000c 3 do. l000C A l300C 3 do. 1300c Ex 5 do.

Furnace cooling By the above-mentioned temperature raise, the surfaces ofthe silicon carbide particles in the inner and intermediate layers are oxidized to form cristobalite and to allow it to react with tridymite to bond. In the external layer, low-melting-point materials included partially in the chamotte are bonded to cristobalite which is transitioned from tridymite during the temperature raise, thus the external layer is sintered with high silica glass.

A test piece in the form of a slab 450 mm sq. was sintered at 1,300C under the same conditions as mentioned above. After the baking it exhibited no dimensional changes at all. Repetition of heating of the test piece from the inner layer side within a range between 800 and 1,300C and cooling caused no warping. From the practical point of view test pieces were repeatedly heated from the inner layer side to 300C (mold preheating temperature) and 1,300C (pouring temperature) and cooled. Here again no warping took place.

When a mold of more than three layers, for example, a mold of four layer construction is desired due to the size etc., of an article to be molded, the compositions of the layers may be made as shown in the following table 3.

The particle size of the first and second intermediate layers may be the same as that of the intermediate layer shown in table 2.

With a mold according to the present invention. castiron V pulleys each measuring 300 mm in diameter and A-] in size which conformed to the Japanese Industrial Standards were actually cast by pouring the molten metal at l,300C into the mold. The experiments indicated that the mold had an extended service life as compared with conventional refractory molds, without the danger of warping which has often been the case with the known molds. The mold of the invention has another advantage of the controllability of its thermal conductivity. Further, the mold is made of inexpensive materials to a great economic advantage.

Because no such durable mold of clad type has ever been proposed, the mold of the invention can be said quite novel and inventive.

We claim:

1. A durable mold for casting molten metals comprismg:

means to receive molten metal and in which said metal becomes solidified, which means comprises a body portion defining a casting cavity, said body portion comprising at least three different layers, each formed of a nonmetallic refractory material having coefficients of thermal expansion decreasing from the external layer remote from the article to be molded to the internal layer closest to the article to be molded,

said internal layer consisting essentially of silicon carbide, the intermediate layers consisting essentially of mixtures of silicon carbide and chamotte and said external layer consisting essentially of chamotte, each of said layers further containing tridymite as an expansion controlling agent for controlling the coefficients of expansion in small amounts sufficient to control the coefficients of expansion at predetermined levels. 

1. A DURABLE MOLD FOR CASTING MOLTEN METALS COMPRISING: MEANS TO RECEIVE MOLTEN METAL AND IN WHICH SAID METAL BECOMES SOLIDIFIED, WHICH MEANS COMPRISES A BODY PORTION DEFINING A CASTING CAVITY, SAID BODY PORTION COMPRISING AT LEAST THREE DIFFERENT LAYERS, EACH FORMED OF A NON-METALLIC REFRACTORY MATERIAL HAVING COEFFICIENTS OF THERMAL EXPANSION DECREASING FROM THE EXTERNAL LAYER REMOTE FROM THE ARTICLE TO BE MOLDED TO THE INTERNAL LAYER CLOSEST TO THE ARTICLE TO BE MOLDED, SAID INTERNAL LAYER CONSISTING ESSENTIALLY OF SILICON CARBIDE, THE INTERMEDIATE LAYERS CONSISTING ESSENTIALLY OF MIXTURES OF SILICON CARBIDE AND CHAMOTTE AND SAID EXTERNAL LAYER CONSISTING ESSENTIALLY OF CHAMOTTE, EACH OF SAID LAYERS FURTHER CONTAINING TRIDYMITE AS AN EXPANSION CONTROLLING AGENT FOR CONTROLLING THE COEFFICIENTS OF EXPANSION IN SMALL AMOUNTS SUFFICIENT TO CONTROL THE COEFFICIENTS OF EXPANSION AT PREDETERMINED LEVELS. 